Geotechnical characteristics of tailings via lime addition

ABSTRACT

Methods and systems for treating tailings at an elevated pH using lime are disclosed herein. In some embodiments, the method comprises (i) providing a tailings stream comprising bicarbonates and a pH less than 9.0, (ii) adding a coagulant comprising calcium hydroxide to the tailings stream to form a mixture having a pH of at least 11.5 and a soluble calcium level no more than 800 mg/L, and (iii) dewatering the mixture to produce a product having a solids content of at least 40% by weight. In some embodiments, the pH and soluble calcium level of the mixture cause chemical modification of clay materials of the mixture via pozzolanic reactions. In some embodiments, the undrained shear strength of the product increases over a period of time of at least two days.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 62/806,512, filed Feb. 15, 2019, and U.S. ProvisionalPatent Application No. 62/729,955, filed Sep. 11, 2018, the disclosuresof which are incorporated herein by reference in their entireties.

TECHNICAL FIELD

This present disclosure relates to systems and methods for treatingtailings, and more particularly to improving geotechnicalcharacteristics of tailings via lime addition.

BACKGROUND

Dewatering and reclaiming oil sand tailings have proven difficult. Anumber of treatment processes have been proposed but none have been ableto cost effectively meet government regulatory standards. One suchstandard was Alberta Energy Regulator's (AER's) Directive 74, whichtargeted for treated tailings a minimum undrained shear strength of 5kilopascals (kPa) within one year of placement in a Dedicated DisposalArea (DDA). Reclamation of the DDA was to be within 5 years after thecompletion of active deposition and required a minimum shear strength of10 kPA to achieve a trafficable surface. Despite significant research byindustry, Directive 74 proved difficult to meet and was replaced byAER's Directive 85 (Fluid Tailings Management for Oil Sands MiningProjects). Rather than target specific strength levels, Directive 85requires all legacy tailings to be reclaimed by the end of mine life andall new tailings to be reclaimed within ten years of the end of minelife. Meeting these requirements will require new treatment technologiesfor fine tailings that will gain enough strength to be used for landformdevelopment in a short timeframe. At the present time, approximately 1.2billion cubic meters of legacy tailings ponds currently exist that haveclay particles suspended in process water. Though sand and overburdenfrom the mining operations can be used in reclamation efforts of theseponds, the fine clays have been difficult to reclaim because of theirhigh plasticity index. Previous attempts to dewater tailings containingthese fine clays have resulted in treated tailings that have low initialshear strength and show temporary or no permanent strength developmentover time. Accordingly, a need exists to effectively treat suchtailings.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and advantages of the presently disclosed technologymay be better understood with regard to the following drawings.

FIG. 1 is a schematic block diagram of a tailings dewatering system, inaccordance with embodiments of the present technology.

FIGS. 2A and 2B are schematic block diagrams of a tailings dewateringsystem, in accordance with embodiments of the present technology.

FIG. 3 is a flow diagram of a method for dewatering tailings, inaccordance with an embodiment of the present technology.

FIGS. 4A and 4B are graphs showing the effects on peak shear strengthover time of treating tailings via varying amounts of coagulants and/orflocculants, in accordance with embodiments of the present technology.

FIG. 5 is a graph showing the effect of calcium hydroxide concentrationon average undrained peak shear strength of treated tailings over time,in accordance with embodiments of the present technology.

FIG. 6 is a graph showing the effect of calcium hydroxide concentrationon undrained peak and remolded shear strength of treated tailings afterdewatering, in accordance with embodiments of the present technology.

FIG. 7 is a graph showing the effect of coagulants on undrained remoldedshear strength of treated tailings over time, in accordance withembodiments of the present technology.

FIG. 8 is a graph showing the effect of coagulants and/or flocculants onthe plastic limits of treated tailings over time, in accordance withembodiments of the present technology.

FIG. 9 is a graph showing the effect of calcium hydroxide concentrationon the plasticity index of treated tailings over time, in accordancewith embodiments of the present technology.

FIG. 10 is a graph showing the effect of calcium hydroxide concentrationon the composition of treated tailings over time, in accordance withembodiments of the present technology.

FIG. 11 is a graph showing the effect of coagulants and/or flocculantson the clay activity of treated tailings over time, in accordance withembodiments of the present technology.

FIG. 12 is a graph showing the effect of calcium hydroxide concentrationon the specific gravity of treated tailings over time, in accordancewith embodiments of the present technology.

FIG. 13 is a graph showing the effect of calcium hydroxide concentrationon the particle size of treated tailings, in accordance with embodimentsof the present technology.

FIG. 14 is a graph showing the effect of temperature on undrained peakshear strength of treated tailings over time, in accordance withembodiments of the present technology.

A person skilled in the relevant art will understand that the featuresshown in the drawings are for purposes of illustrations, and variations,including different and/or additional features and arrangements thereof,are possible.

DETAILED DESCRIPTION I. Overview

Embodiments of the present disclosure relate to improving strength orgeotechnical characteristics of treated tailings via lime addition.Tailings are often treated with coagulants other than lime, such asgypsum, alum, or calcium chloride, in an effort to dewater the tailingsand produce a cake suitable for storage and/or disposal. However, asdescribed elsewhere herein, treating tailings with these coagulants doesnot sufficiently increase the strength profile of cakes such that theycan be appropriately stored, disposed, and/or meet regulatoryrequirements. Though strengths of treated tailings can increase overtime through settling and consolidation, oil sands tailings consolidateat an extremely low rate, if it all, which has created a barrier forreclamation efforts. For example, treatment of tailings with gypsum,alum, calcium chloride, or combinations thereof, does not increase thetailings' undrained shear strength (e.g., peak, remolded and/or residualshear strength) or undrained shear stress (e.g., peak, remolded and/orresidual shear stress) immediately after treatment or after a period oftime (e.g., 2 days, 7 days, 28 days, or longer) on a substantiallypermanent basis. Instead, the strength profiles of cakes produced usingthese coagulants remain unchanged over time or are increased onlytemporarily. For example, oil sand tailings treated with thesecoagulants can gain strength by drying, but this strength can be lostwhen the treated tailings becomes wet again.

Tailings have also been treated with coagulants including lime. However,unlike embodiments of the present disclosure, such treatment has beenunable to sustain improved strength profiles of the cakes over time(e.g., on a substantially permanent basis) via the chemical formation ofhydraulically cementitious compounds on surfaces of the tailings' claymaterials. This is due in part to one or more of treating the tailings(i) without first removing bicarbonates from the process water, (ii) ata pH level that is too low, and/or (iii) without supplying sufficientcalcium cations to drive the pozzolanic reactions and chemically convertclays of the tailings, thereby preventing pozzolanic activity and otherrelated reactions from occurring.

Embodiments of the present disclosure address at least some of the abovedescribed issues for treating tailings to produce a product withimproved geotechnical and strength characteristics. For example,embodiments of the present disclosure include treating tailings with acoagulant comprising calcium hydroxide to form a first mixture having apH of at least 11.5 and a soluble calcium level of no more than 800 mg/L(e.g., 800 parts per million (ppm)), or in some embodiments no more than100 mg/L. Without being bound by theory, a pH of 11.5 can enable cationexchange to occur, e.g., between the calcium cations of the calciumhydroxide and sodium compounds on the clay materials of the tailings.Chemical reactions between calcium hydroxide and bicarbonates in theprocess water maintain the soluble calcium level below a certainthreshold at this stage of the treatment. Embodiments of the presentdisclosure can further comprise adding a flocculant (e.g., an anionicpolyacrylamide polymer) to the first mixture to form a second mixture.The flocculant can bind to free water molecules of the second mixtureand aid the mechanical separation of the water molecules from theremainder of the second mixture. Embodiments of the present disclosurecan further comprise adding a second coagulant comprising calciumhydroxide to form a third mixture having a pH of at least 12.0 and asoluble calcium level of no more than 800 mg/L. Without being bound bytheory, a pH of at least 12.0 can enable pozzolanic activity within thethird mixture, causing clay materials (e.g., kaolinite, illite, etc.) tobe chemically modified and produce calcium bound hydrates (e.g.,silicate and/or aluminate hydrates) therefrom. In doing so, the claymaterials provided by the tailings can be substantially permanentlymodified to form a cementitious crust or matrix having shear strengthabove a certain threshold (e.g., 2 kilopascals (kPa), 3 kPa, 4 kPa, 5kPa, 6 kPa, or greater. The third mixture may be dewatered via adewatering device to form a product (e.g., a cake) having a solidscontent of at least 40% by weight.

Embodiments of the present disclosure enable the product to haveimproved geotechnical and/or strength characteristics relative toconventional systems and methods for treating tailings. For example, asdescribed elsewhere herein, the product can include an undrained shearstrength that increases over a period of time of at least two days, orin some embodiments 7 days, 14 days, 30 days, 60 days, 120 days, orlonger. In addition to or in lieu of the foregoing, as described indetail elsewhere herein, the product can include other characteristicsthat improve over the period of time, such as plasticity index (i.e.,decreases over time), plastic limit (i.e., increases over time), andparticle size (i.e., increases over time), amongst othercharacteristics.

In the Figures, identical reference numbers identify generally similar,and/or identical, elements. Many of the details, dimensions, and otherfeatures shown in the Figures are merely illustrative of particularembodiments of the disclosed technology. Accordingly, other embodimentscan have other details, dimensions, and features without departing fromthe spirit or scope of the disclosure. In addition, those of ordinaryskill in the art will appreciate that further embodiments of the variousdisclosed technologies can be practiced without several of the detailsdescribed below.

II. Systems and Method for Improving Geotechnical and/or StrengthCharacteristics of Tailings Streams Via Lime Addition

FIG. 1 is a schematic block diagram of a tailings dewatering system 100(“system 100”), in accordance with embodiments of the presenttechnology. As shown in the illustrated embodiment, the system 100includes tailings 103 and a coagulant 105 that are provided to a mixer106. The mixer 106 combines the tailings 103 and coagulant 105 toproduces a mixture 107 that is provided to a dewatering device 118. Asexplained in additional detail below, the dewatering device 118separates the mixture 107 into a first stream 119 (e.g., a product or“cake”) comprising a solids content of at least 40% by weight, and asecond stream 120 comprising release water. The first stream 119 can beprovided to a disposal or containment area (e.g., a pond or diked area)and the second stream 120 may be provided as recycle or effluent to adisposal or containment area.

The tailings 103 can be provided from a tailings reservoir 102 (e.g., apond, diked area, tank, etc.), or directly from another process stream101 (e.g., an extraction process stream, a treatment process stream,etc.) without being routed through the tailings reservoir 102. In someembodiments, the tailings 103 can originate from operations related tooil sands and include the remains of the oil sands operations afterextraction of bitumen content. For example, the tailings 103 can includewhole-tailings (WT), thin fluid tailings (TFT), fluid fine tailings(FFT), hydro-cyclone overflow or underflow, and/or mature fine tailings(MFT). In some embodiments, the tailings 103 can originate from theextraction of minerals (e.g., copper, iron ore, gold and/or uranium),e.g., from mining operations. Similar to oil sands tailings, tailingsfrom mining operations contain clay materials that be dewatered andstrengthened through pozzolanic reactions with calcium hydroxide.Additionally or alternatively, treatment with calcium hydroxide hasother benefits such as pH adjustment, bicarbonate removal, heavy metalsremoval, and the treatment of sulfur and other impurities originatingfrom mineral tailings.

The tailings 103 can have a pH less than about 10.0, 9.0, or 8.0, orfrom about 7.0-10.0, 7.5-9.5, or 8.0-9.0. The composition of thetailings 103 can include water (e.g., extraction water), sand,bicarbonates (e.g., sodium bicarbonate), sulfates, clay (e.g.,kaolinite, illite, etc.), residual bitumen particles, and otherimpurities that are suspended in the water. In some embodiments, thetailings 103 can include a solids content of from about 5-40%, a bitumencontent from about 0-3%, and/or a clay content from about 40-100%. Thetailings 103 can be obtained as a batch process (e.g., intermittentlyprovided from tailings ponds) or as a steady-state extraction process(e.g., continuously provided from oil sands or mining operations, orstepwise feeding in pattern). In some embodiments, the tailings 103 mayundergo upstream processing prior to the tailings reservoir 102, e.g.,cyclone separation, screen filtering, thickening and/or dilutionprocesses. The tailings 103 entering the mixer 106 may also be dilutedto decrease the solids content thereof.

The coagulant 105 can include lime and/or inorganic materials thatprovide divalent cations (e.g., calcium), and may be provided from acoagulant reservoir 104 (e.g., a tank). The lime can include hydratedlime (e.g., calcium hydroxide (Ca(OH)₂) and/or slaked quicklime (e.g.,calcium oxide (CaO)). In some embodiments, the hydrated lime can includeenhanced hydrated lime (e.g., calcium hydroxide particles having aspecific surface area of at least 25 m²/g), as described in U.S. patentapplication Ser. No. 15/922,179, now U.S. Pat. No. 10,369,518, filedMar. 15, 2018, the disclosure of which is incorporated herein byreference in its entirety. The lime can be part of a slurry such thatthe lime makes up a portion (e.g., no more than 30%, 25%, 20%, 15%, 10%,or 5% by weight) of the lime slurry. The remainder of the lime slurrycan include water (e.g., release water, makeup water, and/or processwater). In some embodiments, the lime or lime slurry can includedolomitic lime (e.g., lime including at least 25% magnesium oxide on anon-volatile basis), or a combination of quicklime, limestone (e.g.,calcium carbonate (CaCO₃)), hydrated lime, enhanced hydrated lime,dolomitic lime, lime kiln dust, and/or other lime-containing materials.The lime can have a pH of from about 12.0-12.5.

As previously described, the tailings 103 and the coagulant 105 can becombined in the mixer 106 to produce the mixture 107. The mixer 106 canbe a static mixer, a dynamic mixer, or a T-mixer, and/or can includerotatable blades or other means to agitate the combined tailings 103 andcoagulant 105. The residence time in the mixer 106 for the tailings 103and coagulant 105 can be, e.g., less than about 30 seconds, 60 seconds,5 minutes. In some embodiments, the mixer 106 is omitted and thetailings 103 and coagulant 105 can be mixed in-line (e.g., via turbulentflow conditions). In general, the tailings 103 and coagulant 105 aremixed (e.g., via the mixer 106 or in-line) to ensure the mixture 107exiting the mixer 106 has a substantially uniform composition, and adesired pH and/or soluble calcium level. The pH of the mixture 107 canbe at least about 11.5, 11.6, 11.7, 11.8, 11.9, 12.0, 12.1, 12.2, 12.3,12.4 or 12.5. In some embodiments, the pH of the mixture is within arange of 11.5-12.0. Additionally or alternatively, the soluble calciumlevel (i.e., the calcium cations in solution) of the mixture 107 is nomore than 800 mg/L, 750 mg/L, 700 mg/L, 650 mg/L, 600 mg/L, 550 mg/L,500 mg/L, 450 mg/L, 400 mg/L, 350 mg/L, 300 mg/L, 250 mg/L, 200 mg/L,150 mg/L, 100 mg/L, 90 mg/L, 80 mg/L, 70 mg/L, 60 mg/L, 50 mg/L, 40mg/L, or 30 mg/L. In some embodiments, the soluble calcium level of themixture is within a range of 10 mg/L-100 mg/L. As explained inadditional detail elsewhere herein (e.g., with reference to FIG. 2A),the soluble calcium level of the mixture 107 is in part dependent on thepH of the mixture and the bicarbonates present in the tailings 103,which react with the calcium ions and reduce the free calciumconcentration. In some embodiments, a pH of from 11.5 to 12.0 enablesion exchange to occur between the tailings 103 and coagulant 105, andcan aid in minimizing the bicarbonates present in the mixture 107. Inpractice, the pH of the mixture 107 can be measured, e.g., at the outletof the mixer 106, and used to control the pH and/or soluble calciumlevel of the mixture 107 by (i) increasing or decreasing the feed rateof the incoming coagulant 105, and/or (ii) increasing or decreasing theresidence time of the tailings 103 and coagulant 105 in the mixer 106.

As shown in FIG. 1, the system 100 can further include a control system130 to control operations associated with the system 100. Manyembodiments of the control system 130 and/or technology described belowmay take the form of computer-executable instructions, includingroutines executed by a programmable computer. The control system 130may, for example, also include a combination of supervisory control anddata acquisition (SCADA) systems, distributed control systems (DCS),programmable logic controllers (PLC), control devices, and processorsconfigured to process computer-executable instructions. Those skilled inthe relevant art will appreciate that the technology can be practiced oncomputer systems other than those described herein. The technology canbe embodied in a special-purpose computer or data processor that isspecifically programmed, configured or constructed to perform one ormore of the computer-executable instructions described below.Accordingly, the term “control system” as generally used herein refersto any data processor. Information handled by the control system 130 canbe presented at any suitable display medium, including a CRT display orLCD.

The technology can also be practiced in distributed environments, wheretasks or modules are performed by remote processing devices that arelinked through a communications network. In a distributed computingenvironment, program modules or subroutines may be located in local andremote memory storage devices. Aspects of the technology described belowmay be stored or distributed on computer-readable media, includingmagnetic or optically readable or removable computer disks, as well asdistributed electronically over networks. Data structures andtransmissions of data particular to aspects of the technology are alsoencompassed within the scope of particular embodiments of the disclosedtechnology.

FIGS. 2A and 2B are schematic block diagrams of a tailings dewateringsystem (“system 200”), in accordance with embodiments of the presenttechnology. The system 200 includes components and elements similar oridentical to those described with reference to FIG. 1. For example, thesystem 200 includes the previously described tailings 103, coagulant 105(e.g., first coagulant), mixer 106 (e.g., first mixer 106), and mixture107 (e.g., first mixture).

Combining the first coagulant 105 (e.g., calcium hydroxide) with thetailings 103 (e.g., in the first mixer 106 or in-line) increases the pHof the tailings 103 to be at least about 11.5. At or below a pH of 11.5,bicarbonates present in the tailings 103 are substantially depleted dueto reactions with the calcium hydroxide. In doing so, the solublecalcium ions needed for cation exchange within the first mixture 107,e.g., between the calcium cations and bicarbonates provided by thetailings 103, are reduced. Without being bound by theory, such a pH canalso enable the first coagulant 105 to alter the surface charges of theclay of the tailings 103, which promotes dewatering thereof. Using acoagulant other than calcium hydroxide, such as alum (Al₂(SO₄)₃)),gypsum (CaSO₄.2H₂O) and/or calcium chloride (CaCl₂)), to treat thetailings 103 would not enable the clay of the tailings 103 to releasewater in the same manner as calcium hydroxide would. Reactions betweenalum, gypsum, and/or calcium chloride and the clay would not producehydroxides and/or a mixture having a pH of at least 11.5. Instead,treating the tailings stream with alum, for example, would producehydrogen ions (e.g., as sulfuric acid) and generally result in a mixturehaving a pH less than 9.0. As explained in detail elsewhere herein, sucha low pH would preclude pozzolanic reactions from occurring and therebyprevent chemically modifying the tailings 103 to produce a cake withinsufficiently high shear strength. Additionally or alternatively,treating the tailings stream with alum, gypsum, calcium chloride, orother coagulants other than calcium hydroxide would not (i) provide thenecessary pH (e.g., a pH of at least about 11.5) to solubilize silicatesand aluminates of the tailings stream, and/or (ii) supply the necessarysoluble calcium ions for pozzolanic reactions to occur.

Adding the first coagulant 105 including calcium hydroxide to thetailings 103 can cause or enable Reactions 1-4 below to occur within thefirst mixture 107.Ca(OH)_(2(aq))+NaHCO_(3(aq))→→CaCO_(3(aq))+NaOH_((aq)) +H ₂ O  (Reaction1)NaOH_((aq))+NaHCO_(3(aq)) →Na ₂ CO _(3(aq)) +H ₂ O  (Reaction 2)Ca(OH)_(2(aq)) +Na ₂ CO _(3(aq))→CaCO_(3(aq))+2NaOH_((aq))  (Reaction 3)Ca(OH)_(2(aq)) →Ca ²⁺ _((aq))+2OH _((aq))  (Reaction 4)

Per Reaction 1, when sodium bicarbonate (NaHCO₃) of the tailings 103 isexposed to calcium hydroxide (Ca(OH)₂), calcium cations (Ca′) bond withcarbonate ions (CO₃ ²⁻) and sodium bicarbonate is converted to calciumcarbonate (CaCO₃) (also referred to herein as “calcite”), sodiumhydroxide (NaOH) and water (H₂O). Per Reaction 2, the produced sodiumhydroxide from Reaction 1 reacts with sodium bicarbonate to producesodium carbonate (Na₂CO₃) and water. Per Reaction 3, calcium hydroxideof the first coagulant 105 reacts with the produced sodium carbonatefrom Reaction 2 to produce calcium carbonate and sodium hydroxide. PerReaction 4, and as a result of the pH of the first mixture 107 being ator above about 11.5, calcium hydroxide will also readily solubilize toform calcium cations and sodium hydroxide.

In practice, Reactions 1 and 3 are limited only by the availability ofcarbonate ions in the first mixture (i.e., provided by the tailings). Assuch, Reactions 1 and 3 will reduce the amount of soluble calciumcations available for cation exchange (and pozzolanic reactions) tooccur. Stated differently, Reactions 1 and 3 limit the amount of freecalcium cations available to react with clays in the first mixture untilthe carbonate ions are largely depleted and/or removed from the firstmixture. As a result of Reactions 1-4, in some embodiments the firstmixture has a soluble calcium level of no more than 100 mg/L, 90 mg/L,80 mg/L, 70 mg/L, 60 mg/L, 50 mg/L, 40 mg/L, or 30 mg/L.

As shown in FIG. 2A, the first mixture 107 can be combined with aflocculant 109, e.g., from a flocculant reservoir 110 (e.g., a tank orreservoir). The flocculant 109 can be combined with the first mixture107 in-line and/or in a thickener vessel 108 (e.g., a tank orreservoir). The vessel 108 can form, via separation of the first mixture107, (i) a second mixture 111 including a thickened composition havingless water content than that of the first mixture 107, and (ii) processwater 112. Without being bound by theory, separation of the firstmixture 107 into the second mixture 111 and the process water 112 ispromoted at least in part by the pH of the first mixture 107 being atleast 11.5 and/or the coagulant 105 including calcium hydroxide whichalters the surface charges of the clay of the tailings 103 to promotedewatering.

The second mixture 111 can include similar solid minerals, pH andsoluble calcium level to that of the first mixture 107. The processwater 112 can be routed to a separate process (e.g., for bitumenextraction), while the second mixture 111 is routed to furtherdownstream processing. By separating the second mixture 111 and processwater 112, the vessel 108 decreases the amount of water in the secondmixture 111 and the overall volume to be processed by downstreamequipment such as the dewatering device 118. Accordingly, a highervolume of the tailings 103 can be processed by the system 200 relativeto systems that do not remove the process water 112 in such a manner.Additionally, separation of the second mixture 111 and process water 112from one another can decrease overall cycle time of the system 200.

The process water 112 can include hydroxides (e.g., sodium hydroxide),bicarbonates from the tailings 103, and/or other compounds formed asbyproducts of reacting the coagulant 105 with the tailings 103. As shownin FIG. 2A, the process water 112 can be used as a dilutant, e.g., bycombining the process water 112 with the coagulant 105 to form the limeslurry previously described. Additionally or alternatively, as shown inFIG. 2B, the process water 112 can be directed toward and used topromote bitumen extraction, e.g., by combining the process water 112with other process water 126. In some extraction processes for oil sandsoperations, the process water 126 can be supplemented/treated withsodium particles (Na⁺) to aid the release of bitumen from the oil sandsore. Accordingly, one advantage of routing the process water 112 totreat or mix with the process water 126 is the ability to decrease anysupplement addition of sodium particles. Additionally, since the processwater 112 is at least slightly alkaline due to the excess hydroxide ionspresent therein, recycling the process water 112 to the extractionprocess can increase the pH of the oil sand ore and thereby improvebitumen extraction efficiency for the system 200. Yet another advantageof recycling the process water 112 is that heat is already present inthe process water 112, and thus recycling it may require less downstreamheating requirements compared to using just the process water 126. Yetanother advantage of recycling the process water 112 is removing thevolume of the process water 112 from the second mixture 111, whichincreases the solids content of the second mixture 111 and minimizes theoverall volume of material that needs to be dewatered, e.g., viadewatering device 118. This decrease in volume can increase overallthroughput of the system 200, thereby decreasing time and costsassociated with operating the dewatering device 118.

The flocculant 109 can include one or more anionic, cationic, nonionic,or amphoteric polymers, or a combination thereof. The polymers can benaturally occurring (e.g., polysaccharides) or synthetic (e.g.,polyacrylamides). In some embodiments, the flocculant 109 can be addedas a part of a slurry, which may include less than 1% (e.g., about 0.4%)by weight of the flocculant 109, with the substantial remainder beingwater (e.g., process water, release water, and/or makeup water). In someembodiments, at least one component of the flocculant 109 will have ahigh molecular weight (e.g., up to about 50,000 kilodaltons). In someembodiments, the flocculant 109 will have a low molecular weight (e.g.,below about 10,000 kilodaltons). As described in detail elsewhereherein, the flocculant 109 can promote thickening (e.g., increasing thesolids content) of the second mixture 111, e.g., by forming bonds withcolloids in the vessel 108, e.g., that were originally provided via thetailings 103. That is, the flocculant 109 can bond with the clay presentin the tailings 103 to form a floc that is physically removed from therest of the mixture. In doing so, the flocculant 109 also aids themechanical separation of free water from the mixture. In someembodiments, the amount of flocculant 109 added to the first mixture 107is based at least in part on solids content of the second mixture 111and/or process water 112. For example, the flocculant 109 may be addedto the mixture 107 and/or vessel 108 such that (i) the solids content ofthe second mixture 111 is greater than a predetermined threshold (e.g.,30%) and/or (b) solids content of the process water 112 is less than apredetermined threshold (e.g., 3%). That is, if the second mixture 111has a solids content less than 30% solids by weight, the amount offlocculant 109 added to the first mixture 107 and/or vessel 108 may beincreased, and/or if the process water 112 has a solids content greaterthan 3% solids by weight, the amount of flocculant 109 added to themixture 107 and/or vessel 108 may be increased.

As shown in FIG. 2A, the second mixture 111 can be combined with asecond coagulant 115 in a second mixer 116 to form a third mixture 117.The second coagulant 115 can be provided from a coagulant reservoir 114(e.g., the coagulant reservoir 104) and can be similar or identical tothe first coagulant 105 previously described. Accordingly, the secondcoagulant 115 may include lime and be a lime slurry such that the limemakes up a portion (e.g., no more than 30%, 25%, 20%, 15%, 10%, or 5% byweight) of the lime slurry. The second mixer 116 can be identical orsimilar to the first mixer 106 previously described.

Adding the second coagulant 115 to the second mixture 111 increases thepH and soluble calcium level (i.e., the amount of calcium cationspresent) in the third mixture (e.g., via Reaction 4). The increase inthe soluble calcium level of the third mixture relative to that of thefirst and second mixtures is due in part to the removal of bicarbonatesvia Reactions 1 and 2 that previously occurred after the first coagulant105 was added to the first mixer 106. As such, the additional calciumcations provided via the second coagulant 115 result in a higher solublecalcium level since the calcium ions are not being consumed by thebicarbonates, which are no longer present or are present in smallerquantities relative to the first and second mixtures. The third mixturecan have a pH of at least 11.8, 11.9, 12.0, 12.1, 12.2, 12.3, 12.4, or12.5, and a soluble calcium level of no more than 300 mg/L, 400 mg/L,500 mg/L, 600 mg/L, 700 mg/L or 800 mg/L. In some embodiments, the pH ofthe third mixture is within a range of from about 12.0-12.5, and thesoluble calcium level of the third mixture is within a range of fromabout 300 mg/L-800 mg/L, 300 mg/L-700 mg/L, 400 mg/L-600 mg/L, 450mg/L-550 mg/L, or other incremental ranges between these ranges. As aresult of adding the second coagulant 115 including calcium hydroxide tothe second mixture 111, or more specifically providing additionalcalcium cations and increasing the pH to be at least 12.0, pozzolanicactivity can occur via one or both of Reactions 5 and 6.Ca(OH)₂ +Si(OH)₄ →CaH ₂ SiO _(4 ·)2H ₂ O  (Reaction 5)Ca(OH)₂ +Al(OH)₄ →CaH ₂ AlO _(4 ·)2H ₂ O  (Reaction 6)

Per Reaction 5, calcium cations of the second coagulant 115 react withsilicic acid (Si(OH)₄) functional groups of the clay (e.g., kaolinite(Al₂Si₂O₅(OH)₄) or illite (K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂,(H₂O)])provided via the tailings 103 to produce calcium silicate hydrates(CaH₂SiO₄.2H₂O). Per Reaction 6, calcium cations of the second coagulant115 react with aluminate (Al(OH)₄) functional groups of the clayprovided via the tailings 103 to produce calcium aluminum hydrates(CaH₂SiO₄.2H₂O). In addition to Reactions 5 and 6, calcium cationsprovided via the second coagulant 115 can replace cations (e.g., sodiumand potassium) on the surface of the clay provided via the tailings 103.Pozzolanic reactions (e.g., Reactions 5 and 6) will only occur in anenvironment having a pH of at least about 11.8, 11.9, or 12.0. Withoutbeing bound by theory, this is because such a pH increases thesolubility of silicon and aluminum ions to be sufficiently high andprovide the driving force for the pozzolanic reactions to occur.

As a result of Reactions 5 and 6, the stability of the clay ischemically modified. This chemical modification of the clay can cause(i) the particle size of the clay to increase, and (ii) the water layerof the clay particles to generally decrease. Furthermore, as explainedin detail elsewhere herein, the produced calcium silicate hydratesand/or calcium aluminum hydrates exhibit properties associated with acementation matrix that are substantially irreversible. Generallyspeaking, the pozzolanic reactions therefore increase the shear strengthof the third mixture and the downstream product streams.

The previously described pozzolanic reactions will generally not occurfor tailings that are treated with coagulants, such as alum, gypsum, andcalcium chloride, that do not provide the chemical environment describedabove. For tailings treated with gypsum or calcium chloride, forexample, the calcium cations will generally solubilize at a lower pH(i.e., less than 11.5) and their addition to tailings will not increasethe pH (e.g., above 12.0) of the treated mixture. For tailings treatedwith alum (Al₂(SO₄)₃), sulfuric acid is produced which activelydecreases pH of the treated mixture. As a result of not having asufficiently high pH to drive the pozzolanic reactions, the chemicalmodification of the clay resulting from the pozzolanic reactions willnot occur when tailings are treated with these compounds. As such, theshear strength of the resulting mixture and downstream products may beless than that of tailings treated with calcium hydroxide according toembodiments of the present technology. Furthermore, treating tailingswith alum, gypsum, and/or calcium chloride is unable to produce overtime the chemically modified cementitious crust that embodiments of thepresent technology are able to produce.

An advantage of the adding the first coagulant 105, flocculant 109, andsecond coagulant 115 in a step-wise manner, as opposed to adding only asingle coagulant, is the decreased cycle time of the overall system 200.That is, adding the flocculant 109 (after adding the first coagulant105) to the vessel 108 allows the flocculant 109 to flocculate thesolution in the vessel 108 without the significant presence of solublecalcium ions, which results in a more desirable floc formation andimproved settling of solids in the second mixture 111. Additionally,since the second coagulant 115 is combined with the second mixture 111after removing bicarbonates (e.g., via the second stream 112), thebicarbonates do not limit the effectiveness of the second coagulant 115to promote pozzolanic reactions, as may be the case if only a singlelime dosage was used.

As further shown in FIG. 2A, the third mixture 117 is conveyed (e.g.,via gravity and/or a pump) from the second mixer 116 to the dewateringdevice 118 or other treatment processes, e.g., via a dewatering devicebypass. The other treatment processes can include, e.g., thin liftdeposition, thick lift deposition, deep deposition, or water-cappingtechnologies. The dewatering device 118 can include a centrifuge, afiltration system and/or other similar features, components or systemsthat provide a physical force on the second mixture 117 to promotedewatering, e.g., by separating the second mixture 117 into the firststream 119 (e.g., a product or “cake”) and the second stream 120 (e.g.,a centrate or a filtrate). The centrifuge can include a scrollcentrifugation unit, a solid bowl decanter centrifuge, screen bowlcentrifuge, conical solid bowl centrifuge, cylindrical solid bowlcentrifuge, a conical-cylindrical solid bowl centrifuge, or othercentrifuges used or known in the relevant art. The filtration system caninclude a vacuum filtration system, a pressure filtration system, beltfilter press, or other type of filtering apparatus known in the relevantart that utilizes a desired filtration process. In some embodiments, thefiltration system can include a Whatman 50, 2.7 micron filter or similarcomponent or system that can subject the second mixture 117 to at leastabout 100 psig of air pressure.

The third mixture 117 may be transferred to the centrifuge or filterimmediately after mixing in the second mixer 116 (e.g., based on ameasured composition taken at an outlet of the second mixer 116) orafter a predetermined period of time. In some embodiments, the residencetime of the third mixture 117 in the second mixer 116 may be less than 5minutes, 30 minutes, or one hour. In some embodiments, the third mixture117 may be retained for more than one hour, e.g., one day, one week, onemonth, or longer. In general, the third mixture 117 may be retained forany desired amount of time to ensure it has been sufficiently modifiedfor the dewatering device 118 to separate a sufficient or optimal amountof water from the solids of the third mixture 117.

The dewatering device 118 has a first outlet that receives the firststream 119, and a second outlet that receiver the second stream 120. Asexplained in more detail elsewhere herein (e.g., with reference to FIGS.4A-14), the first stream 119 can be a solid, soft solid, cake, orpumpable fluid material composed of the particulate matter provided viathe tailings 103, such as sand, silt, (chemically modified) clay, andresidual bitumen, as well as soluble calcium ions provided via the firstand second coagulants 105, 115. The first stream 119 can include asolids content of at least 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%,85%, 90%, or 95% by weight. More generally, the first stream 119 mayinclude a greater percentage of solids by weight than the percentage ofliquids by weight. Characteristics (e.g., geotechnical characteristics)of the first stream are described in additional detail herein, e.g.,with reference to FIGS. 4A-14. The first stream 119 may be provided toan external site (e.g., a pond, diked area, temporary storage, and/orreclamation area) via a pump, belt, truck, and/or other conveyingsystem(s). In some embodiments, the mixture 117 can be placed on one ormore pads in thin/thick lifts to consolidate and dry the solids contentcontained therein.

The second stream 120 can include a solids content less than 10%, 5%,4%, 3%, 2%, or 1% by weight. The solids content may include particulatematter such as sand, silt, clay, carbonates, residual bitumen, and/orcalcium ions. The second stream 120 can be directed to a pond and/or beused as recycle 122. As shown in FIG. 2A, the recycle 122 can becombined with (a) the tailings reservoir 102 via line 122 a, (b) thetailings 103 via line 122 b, (c) the coagulant reservoir 104 via line122 c, (d) the first coagulant 105 via line 122 d, (e) the coagulantreservoir 114 via line 122 e, and/or (f) the second mixture 117 via line122 f. Advantageously, combining the recycle 122 with the tailings 103can increase the pH of the tailings 103, which can enable solublecalcium cations of the recycle 122 to react with bicarbonates present inthe tailings 103 and thereby form insoluble compounds that precipitateout of solution and separate from the tailings 103. Reducing the amountof bicarbonates in the tailings 103 can reduce the amount of the firstand second coagulants 105, 115 needed for enhanced dewatering to occur,which in turn can reduce operation costs for the system 200. In someembodiments, the second stream 120 may also be treated with carbondioxide to reduce the pH and/or the amount of soluble calcium cations ofthe second stream 120. This can be done via natural absorption ofbicarbonates, e.g., by reacting the bicarbonates with carbon dioxidepresent in the atmosphere, or by actively injecting carbon dioxide intothe second stream 120. In such embodiments, the reaction may produce abuffer layer comprising calcium carbonate or bicarbonates on top (e.g.,on an outer surface) of the second stream 120, effectively forming aseal.

The system 200 can include the control system 130, as previouslydescribed. The control system 130 can be used to control operation ofthe system 200. For example, the control system 130 can control (e.g.,regulate, limit and/or prevent) the flow of fluids (e.g., tailings 103,first coagulant 105, first mixture 107, flocculant 109, second mixture111, second coagulant 115, third mixture 117, first stream 119, secondstream 120, recycle 122, etc.) to and/or from different units (e.g.,tailings reservoir 102, coagulant reservoir 104, first mixer 106, vessel108, flocculant reservoir 110, second mixer 116, dewatering device 118,etc.) of the system 200. Additionally, the control system 130 cancontrol operation of individual units (e.g., the first mixer 106, secondmixer 116, dewatering device 118, etc.).

FIG. 3 is a flow diagram of a method 300 for dewatering tailings, inaccordance with embodiments of the present technology. The method 300includes providing tailings (e.g., the tailings 103; FIGS. 1 and 2A)having bicarbonates and a pH less than 9.0 (process portion 302), andadding a first coagulant (e.g., the first coagulant 105; FIGS. 1 and 2A)including calcium hydroxide to the tailings to form a first mixture(e.g., the first mixture 107; FIGS. 1 and 2A) (process portion 304). Forembodiments in which the tailings are provided as a continuous flow orstream, the coagulant may be added as a continuous flow or stream, andfor embodiments in which the tailings are provided in batches, thecoagulant may be added in individual batches. Adding the first coagulantincluding calcium hydroxide to the tailings can cause the pH of thetailings to increase to be at least about 11.5, and cause Reactions 1-4(previously described) to occur within the first mixture.

The method 300 further includes combining the first mixture with aflocculant (e.g., the flocculant 109; FIG. 2A) to produce a secondmixture (e.g., the second mixture 111; FIG. 2A) and process water (e.g.,process water 112; FIG. 2A) (process portion 306). As explainedelsewhere herein, the flocculant can react with clay colloids to form afloc, which can be physically removed along the entrained water (e.g.,free water and water molecules produced via Reactions 1 and 2) andpromote the mechanical separation of the clay colloids from the mixture.In doing so, the first mixture can separate into the second mixture andthe process water.

The method 300 further includes separating or removing the process waterfrom the second mixture (process portion 308). As explained elsewhereherein, this can be done by conveying the second mixture to a downstreamcontainer or mixer (e.g., the second mixer 116; FIG. 2A) and/or removingthe process water from a vessel (e.g., the thickener vessel 108; FIG.2A) containing the second mixture and process water. As a result ofReactions 1-4 and removing the process water from the second mixture,the second mixture may include less bicarbonates than the first mixture.

The method 300 further comprises adding a second coagulant (e.g., thesecond coagulant 115; FIG. 2A) including calcium hydroxide to the secondmixture to produce a third mixture (e.g., the third mixture 117; FIG.2A) (process portion 310). As described elsewhere herein, adding thesecond coagulant including calcium hydroxide to the tailings, or morespecifically, providing additional calcium cations and increasing the pHto at least 12.0, enables pozzolanic activity to occur, e.g., viaReactions 5 and/or 6.

The method 300 further includes dewatering the third mixture to producea first stream (e.g., the first stream 119; FIG. 2A) having a solidscontent of at least 40% by weight, and a second stream (e.g., the secondstream 120; FIG. 2A) have a solids content less than 10% by weight.Dewatering the third mixture can occur via a dewatering device (e.g.,the dewatering device 118; FIG. 2A). The first stream may be provided toan external site (e.g., a pond, diked area, temporary storage, and/orreclamation area) via a pump, belt, truck, and/or other conveyingsystem(s). As explained in additional detail herein, pumping the firststream to the external site can shear the first stream and thereby causeresuspension of the solid minerals of the first stream originallyprovided via the tailings. As explained in more detail elsewhere herein,the first stream can have an undrained shear strength and/or shearstress that increases over a period of time (e.g., 1 day, 2 days, 3days, 4 days, 5 days, 6 days, 1 week, 2 weeks, 3 weeks, 1 month, 2months, 3 months, 6 months, 1 year, or longer). After dewatering (e.g.,less than 1 day after dewatering), the undrained shear strength (e.g.,peak, average, remolded, or residual undrained shear strength) and/orshear stress (e.g., peak, average, remolded, or residual undrained shearstress) for the third mixture and/or second stream can be, e.g., atleast 200 Pa, 500 Pa, 1 kPa, 2 kPa, 2.5 kPa, 3.0 kPa, 3.5 kPa, 4.0 kPa,4.5 kPa, 5.0 kPa, 5.5 kPa, 6.0 kPa, 6.5 kPa, or 7.0 kPa, as explained indetail elsewhere herein (e.g., with reference to FIGS. 4A-14).Additionally, after dewatering (e.g., more than 1 day after dewatering),the undrained shear strength and/or shear stress for the third mixtureand/or second stream can be, e.g., at least 5 kPa, 10 kPa, 20 kPa, 30kPa, 40 kPa, 50 kPa, 60 kPa, 70 kPa, 80 kPa, 90 kPa, 100 kPa, or 110kPa. The lower initial shear strength and/or shear stress can bebeneficial, as this allows the third mixture and/or second stream to bepumpable, e.g., from the centrifuge to a containment area, as describedwith reference to FIG. 2A.

III. Experimental Data and Examples

FIGS. 4A-14 show results of examples and tests that corroborate theembodiments described above. The results shown in FIGS. 4A-14 relate toenhanced geotechnical or strength characteristics and correspond totreated tailings streams. The treated tailings streams can correspond tothe second mixture 111, third mixture 117, and/or first stream 119 (FIG.2A) unless noted otherwise. For the results of FIGS. 4A-14, theundrained peak and residual shear strengths of the pressure filtrationand/or centrifuge samples (e.g., cakes) were measured via a BrookfieldRST-SST rheometer. The samples were deformed at a constant rotationalspeed of 0.1 revolutions per minute for 15 minutes using a vanemeasuring system. The cakes produced were placed into 8 mm diameter jarsand levelled to obtain a smooth surface. A VT-20-10 spindle (i.e., aspindle with a 20 mm height and 10 mm diameter) was used to measureundrained shear strengths less than 10 kPa (e.g., for the results ofFIGS. 4A and 6), and a VT-10-5 spindle (i.e., a spindle with a 10 mmheight and 5 mm diameter) was used to measure undrained shear strengthsat or above 10 kPa (e.g., for the results of FIG. 4B). The undrainedpeak shear strength of the samples corresponds to the maximum shearstress recorded during the test (e.g., for the results of FIGS. 4A-5).The undrained remolded shear strength of the samples corresponds to theshear stress retained by the samples post failure (e.g., by shear)(e.g., for the results of FIG. 7). The average undrained peak orremolded shear strength corresponds to the mean value of multiple datapoints obtained during each undrained shear strength measurement (e.g.,for the results of FIGS. 5 and 14). Test methods for determining theshear strength of soils may also correspond to the test methodsdescribed in Standard ASTM D5321/D5321M.

FIGS. 4A and 4B are graphs showing the effects on peak undrained shearstrength over time of treated tailings using varying coagulants and/orflocculants, in accordance with embodiments of the present technology.The peak undrained shear strength can be defined as the maximum value ofthe shear stress measured in an undrained system, and can generally beused to understand the shear stress a given solution or product cansustain before failing. For the tests conducted for FIGS. 4A and 4B,tailings samples were treated using (a) 0 ppm coagulant or flocculant(i.e., a control group), (b) 4000 ppm calcium hydroxide on a wet weightbasis, (c) 10000 ppm calcium hydroxide on a wet weight basis, (d) 1500ppm A3338 polymer (i.e., an anionic polyacrylamide polymer) on a drysolids basis, and (e) a combination of 700 ppm alum on a wet weightbasis and 1500 ppm A3338 polymer on a dry solids basis. The undrainedpeak shear strength was measured for each of the treated tailingssamples at 0, 1, 7, and 28 days after treatment. The treated tailingssamples were sheared in a cylinder at a shear rate, and the peak shearstrength was calculated based on the shear rate and viscosity of thetreated tailings.

The results shown in FIG. 4A correspond to treated tailings sampleshaving about 55% solids content by weight. As shown in FIG. 4A, thetreated tailings samples corresponding to the 4000 ppm and 10000 ppmcalcium hydroxide treated samples were the only samples that exhibited acontinuous increase in undrained peak shear strength over time. That is,the undrained peak shear strength of the 4000 ppm calcium hydroxidetreated sample increased from about 2.3 kPa at day 0, to 2.6 kPa at day1, to 3.9 kPa at day 7, to 5.3 kPa at day 28. The undrained peak shearstrength of the 10000 ppm calcium hydroxide treated sample exhibitedrelatively higher shear strength, exhibiting about 2.6 kPa at day 0, 3.5kPa at day 1, 4.5 kPa at day 7, and 6.2 kPa at day 28. As also shown inFIG. 4A, the control sample exhibited an overall decrease in undrainedpeak shear strength, the 1500 ppm A3338 treated sample exhibited aslight decrease in undrained peak shear strength from day 1 to day 7,and the 700 ppm alum and 1500 ppm A338 treated sample exhibited a firstdecrease in undrained peak shear strength from day 0 to day 1 andanother decrease in undrained peak shear strength from day 7 to day 28.

The 4000 ppm and 10000 ppm calcium hydroxide treated tailings samplesboth have a pH above 12.0. Such a pH is necessary to solubilize thesilica and/or alumina compounds of the clay such that the silica and/oralumina can react with soluble calcium cations. The clays of thesetreated tailings samples likely were chemically modified via pozzolanicreactions, which may be responsible for the increase in peak shearstrength relative to the other treated tailings samples that were notchemically modified via pozzolanic reactions. The increase in peak shearstrength of the 10000 ppm calcium hydroxide sample relative to the 4000ppm calcium hydroxide sample may be a result of the additional solublecalcium cations present in the 10000 ppm calcium hydroxide treatedsample. As described elsewhere herein (e.g., with reference to FIG. 2A),soluble calcium cations are a necessary driving force for chemicallyconverting (i) silicic acid to calcium silicate hydrates and/or (ii)aluminate to calcium aluminum hydrates via pozzolanic reactions (e.g.,Reactions 5 and 6). Accordingly, the additional calcium cations of the10000 ppm calcium hydroxide treated sample may have enabled additionalsilicic acid and/or aluminate functional groups to be converted tocalcium silicate hydrates and calcium aluminum hydrates respectively,thereby causing the peak shear strength of the 10000 ppm calciumhydroxide treated sample to be higher than that of the 4000 ppm calciumhydroxide treated sample.

The results shown in FIG. 4B correspond to treated tailings sampleshaving about 70% solids content by weight. As shown in FIG. 4B, thetreated tailings samples corresponding to the 4000 ppm and 10000 ppmcalcium hydroxide treated samples were the only samples that exhibited acontinuous increase in undrained peak shear strength over time. That is,the undrained peak shear strength of the 4000 ppm calcium hydroxidetreated sample increased from about 30 kPa at day 0, to 39 kPa at day 1,to 48 kPa at day 7, to 52 kPa at day 28. The undrained peak shearstrength of the 10000 ppm calcium hydroxide treated sample exhibitedrelatively higher undrained peak shear strength, providing about 58 kPaat day 0, 80 kPa at day 1, 100 kPa at day 7, and 102 kPa at day 28. Thecontrol sample exhibited no increase in undrained peak shear strengthbetween days 1 and 7, the 1500 ppm A3338 treated sample exhibited aslight overall decrease in undrained peak shear strength, and the 700ppm alum and 1500 ppm A338 treated sample exhibited no overall increasein undrained peak shear strength from day 0 to day 28.

Comparing the results of FIGS. 4A and 4B with one another, the increasein solids content of the treated samples affects the peak shear strengthof the calcium hydroxide treated samples. That is, the undrained peakshear strengths for the 4000 ppm and 10000 ppm calcium hydroxide treatedsamples are higher for the 70% solids content relative to the 55% solidscontent. Accordingly, the undrained peak shear strength appears to bedirectly correlated to the percent solids content of the calciumhydroxide treated samples.

FIG. 5 is a graph showing the effect of calcium hydroxide concentrationon average peak undrained shear strength of dewatered tailings overtime, in accordance with embodiments of the present technology. For thetests conducted for FIG. 5, dewatered tailings samples, which may bereferred to as “cakes,” having a solids content within a range fromabout 50% to 70% solids were treated using (a) 0 ppm coagulant orflocculant (i.e., a control group), (b) 1500 ppm calcium hydroxide on awet weight basis, (c) 3000 ppm calcium hydroxide on a wet weight basis,and (d) 4000 ppm calcium hydroxide on a wet weight basis.

As shown in FIG. 5, each of the treated tailings samples exhibits anincrease in average undrained peak shear strength over time, with themeasured average undrained peak shear strength for day 180 being thehighest measurement for each of the samples. As such, each of thetreated samples exhibited a continuous increase in average undrainedpeak shear strength over a time period of 180 days. Additionally, theoverall average undrained peak shear strength is directly correlated tothe calcium hydrogen concentration. That is, the overall averageundrained peak shear strength for the 1500 ppm calcium hydroxide treatedsample is higher than that of the control group (i.e., the 0 ppm calciumhydroxide treated sample), the overall average undrained peak shearstrength for the 3000 ppm calcium hydroxide treated sample is higherthan that of the 1500 ppm calcium hydroxide treated sample, and theoverall average undrained peak shear strength for the 4000 ppm calciumhydroxide treated sample is higher than that of the 3000 ppm calciumhydroxide treated sample.

The 3000 ppm and 4000 ppm calcium hydroxide treated samples each have apH above 12.0. Accordingly, the clays of these treated samples likelywere chemically modified via pozzolanic reactions which, without beingbound by theory, are responsible for (i) the increase in averageundrained peak shear strength relative to the other treated tailingssamples, and (ii) the average undrained peak shear strength being above5.0 kPa after day 60. The increase in average undrained peak shearstrength of the 3000 ppm calcium hydroxide sample relative to the 1500ppm and/or 0 ppm calcium hydroxide samples may be a result of thepozzolanic reactions that occurred for the 3000 ppm calcium hydroxidetreated sample. Additionally, the increase in average undrained peakshear strength of the 4000 ppm calcium hydroxide treated sample relativeto the 3000 ppm calcium hydroxide treated sample may be a result of theadditional soluble calcium cations present in the 4000 ppm calciumhydroxide treated sample. As described elsewhere herein (e.g., withreference to FIG. 2A), soluble calcium cations are a necessary drivingforce for chemically converting (i) silicic acid to calcium silicatehydrates and/or (ii) aluminate to calcium aluminum hydrates viapozzolanic reactions (e.g., Reactions 5 and 6). Accordingly, theadditional calcium cations of the 4000 ppm calcium hydroxide treatedsample may enable additional silicic acid and/or aluminate functionalgroups to be converted to calcium silicate hydrates and calcium aluminumhydrates respectively, thereby causing the average undrained peak shearstrength of the 4000 ppm calcium hydroxide treated sample to be higherthan that of the 3000 ppm calcium hydroxide treated sample.

FIG. 6 is a graph showing the effect of calcium hydroxide concentrationon undrained peak and remolded shear strength of treated dewateredtailings after 6 months of curing, in accordance with embodiments of thepresent technology. Remolded shear strength corresponds to the magnitudeof shear stress a treated tailings can sustain after being disturbed inan undrained condition. For the tests conducted for FIG. 6, theundrained remolded shear strength is the shear strength retained by thesamples post failure by shearing the soils using the rotation of a vanespindle at given rates of shear. For the tests conducted for FIG. 6,tailings samples having a solids content within a range from about 50%to 70% solids were treated using (a) 0 ppm coagulant or flocculant(i.e., a control group), (b) 1500 ppm calcium hydroxide on a wet weightbasis, (c) 3000 ppm calcium hydroxide on a wet weight basis, and (d)4000 ppm calcium hydroxide on a wet weight basis. Each of the treatedtailings samples exhibited an increase in peak and remolded shearstrength as the calcium hydroxide concentration was increased. That is,the treated samples indicate a direct correlation between calciumhydroxide concentration and peak and remolded undrained shear strength.Notably, the 3000 ppm and 4000 ppm calcium hydroxide treated sampleseach have a pH above 12.0. Accordingly, the clays of these treatedsamples likely were chemically modified via pozzolanic reactions, whichmay be responsible for (i) the increase in peak and remolded shearstrength relative to the other treated tailings samples, and (ii) thepeak shear strength being above 5000 Pa after day 60.

FIG. 7 is a graph showing the effect of coagulants on remolded shearstrength of treated tailings over time, in accordance with embodimentsof the present technology. For the tests conducted for FIG. 7, tailingssamples were treated using (a) 0 ppm coagulant or flocculant (i.e., acontrol group), (b) 7000 ppm gypsum on a wet weight basis, (c) 4500 ppmcalcium chloride on a wet weight basis, and (d) 3000 ppm calciumhydroxide on a wet weight basis. The coagulant doses were selected toprovide a 1600 ppm supply of calcium. The remolded shear strength foreach of the treated tailings samples was measured at day 1 and day 120after treatment.

As shown in FIG. 7, the samples for the control group, 7000 ppm gypsum,and 4500 ppm calcium chloride exhibited a remolded shear strength below2000 Pa, whereas the 3000 ppm calcium hydroxide treated sample exhibiteda remolded shear strength of about 5900 Pa. The 3000 ppm calciumhydroxide sample had a pH above 12.0. Accordingly, the clays of thistailings sample was likely modified via pozzolanic reactions, which maybe responsible for (i) the increase in remolded shear strength relativeto the other treated tailings samples, and (ii) the remolded shearstrength being above 5000 Pa after day 60.

FIG. 8 is a graph showing the effect of coagulants and/or flocculants onthe plastic limits of treated tailings over time, in accordance withembodiments of the present technology. For the tests conducted for FIG.8, tailings samples were treated using (a) 0 ppm coagulant or flocculant(i.e., a control group), (b) 4000 ppm calcium hydroxide on a wet weightbasis, (c) 10000 ppm calcium hydroxide on a wet weight basis, (d) 1500ppm A3338 polymer on a dry solids basis, and (e) a combination 700 ppmalum on a wet weight basis and 1500 ppm A3338 polymer on a dry solidsbasis. The plastic limits were measured for each of the treated tailingssamples at day 0 and day 28 after treatment. Generally speaking, theplastic limit corresponds to the water content at which a sample beginsto transition from a plastic state to a solid state, or stateddifferently, the plastic limit corresponds to the maximum amount ofmoisture content a sample (e.g., a “cake”) can hold while still behavingas a plastic and not a solid. It is generally desirable for the plasticlimit of a treated tailings sample to increase over time, as thisindicates the geotechnical or strength characteristics have improvedsuch that the sample can transition from a plastic state to a solidstate at higher moisture contents.

As shown in FIG. 8, the 4000 ppm and 10000 ppm calcium hydroxide treatedsamples exhibited the largest percent and overall change in plasticlimit, with the plastic limit for the 4000 ppm calcium hydroxide treatedsample increasing from about 27% to 35%, and the plastic limit for the10000 ppm calcium hydroxide treated sample increasing from about 42% to47%. The control group and alum+A3338 samples exhibited decreases intheir plastic limits, and the A3338 samples exhibited a slight increase.The plastic limit of the A3338 treated sample at day 28 was about 25%,which was less than the plastic limit of the 4000 ppm and 10000 ppmcalcium hydroxide treated samples at day 0.

The 4000 ppm and 10000 ppm calcium hydroxide treated tailings samplesboth have a pH above 12.0. Accordingly, the clay of these treatedtailings samples likely were chemically modified via pozzolanicreactions, which may be responsible for their increase in plastic limitover time relative to the other treated tailings samples that were notchemically modified via pozzolanic reactions. The increase in plasticlimit of the 10000 ppm calcium hydroxide sample relative to the 4000 ppmcalcium hydroxide sample may be a result of the additional solublecalcium cations present in the 10000 ppm calcium hydroxide treatedsample.

FIG. 9 is a graph showing the effect of calcium hydroxide concentrationon the plasticity index of treated tailings over time, in accordancewith embodiments of the present technology. For the tests conducted forFIG. 9, tailings samples were treated using (a) 0 ppm coagulant orflocculant (i.e., a control group), (b) 1500 ppm calcium hydroxide on awet weight basis, (c) 3000 ppm calcium hydroxide on a wet weight basis,and (d) 4000 ppm calcium hydroxide on a wet weight basis. Each of thetreated samples were measured at days 1, 30, 60, and 100. The plasticityindex measures the liquid and plastic limits of a soil, or moreparticularly the difference between the liquid and plastic limits, andtends to be high for soils with clay. It is generally desirable for theplasticity index of a treated tailings sample to decrease over time, asthis indicates that the texture of the clays present in the tailings ismodified, e.g., via (i) coagulation and increase in particle size,and/or (ii) pozzolanic reactions to calcium silicate hydrates and/orcalcium aluminum hydrates.

As shown in FIG. 9, the plasticity index decreases over time for the1500 ppm, 3000 ppm, and 4000 ppm calcium hydroxide treated samples. Thecontrol group shows generally no overall change or a slight overallincrease in plasticity index from about 44% on day 1 to about 45% on day100. Notably, the plasticity indexes for the 3000 ppm and 4000 ppmcalcium hydroxide treated samples are significantly less than the 1500ppm calcium hydroxide treated sample. This may be due to pozzolanicreactions chemically modifying the clays of the 3000 ppm and 4000 ppmsamples, as they have a pH above 12.0.

FIG. 10 is a graph showing the effect of calcium hydroxide concentrationon the composition of treated tailings over time, in accordance withembodiments of the present technology. For the tests conducted for FIG.10, tailings samples were treated using (a) 0 ppm coagulant orflocculant (i.e., a control group), (b) 1500 ppm calcium hydroxide on awet weight basis, and (c) 4000 ppm calcium hydroxide on a wet weightbasis. The composition of the samples was measured 60 days aftertreatment to determine the amount of calcite (i.e., calcium carbonate),kaolinite, illite, quartz, and amorphous phase materials. The amorphousphase materials include calcium silicate hydrates, calcium aluminumhydrates, and/or those materials produced as a result of cementitious orpozzolanic reactions.

As shown in FIG. 10, as more calcium hydroxide was added to the tailingssamples (i) the composition of calcite and amorphous phase materialsincreased, and (ii) the composition of kaolinite and illite decreased.The composition of quartz remained generally constant for each of thesamples, which is expected since quartz is generally not reactive withcalcium hydroxide. Per Reactions 1-4, described elsewhere herein (e.g.,with reference to FIG. 2A), the increase in calcite indicates theconversion of bicarbonates of the tailings via reaction with calciumcations from the calcium hydroxide. Per Reactions 5 and 6, describedelsewhere herein (e.g., with reference to FIG. 2A), the decrease inkaolinite and illite and increase in amorphous phase materials indicatethe conversion of kaolinite and illite (and other clays not shown inFIG. 10) to amorphous phase materials. Moreover, the relativelysignificant decrease in kaolinite and illite for the 4000 ppm treatedsample, relative to the 1500 ppm relative sample, indicates the effectof pozzolanic reactions since only the 4000 ppm treated sample had a pHof at least 12.0. For the 4000 ppm and 1500 ppm treated samples, thismay explain the about (i) 50% decrease in kaolinite, (ii) 75% decreasein illite, and (iii) 300% increase in amorphous phase materials.

FIG. 11 is a graph showing the effect of coagulants and/or flocculantson the clay activity of treated tailings over time, in accordance withembodiments of the present technology. For the tests conducted for FIG.11, tailings samples were treated using (a) 0 ppm coagulant orflocculant (i.e., a control group), (b) 4000 ppm calcium hydroxide on awet weight basis, (c) 10000 ppm calcium hydroxide, (d) 1500 ppm A3338polymer on a dry solids basis, and (e) a combination of 700 ppm alum ona wet weight basis and 1500 ppm A3338 polymer on a dry solids basis. Thecolloidal clay activity was measured for each of the treated tailingssamples at 0 and 7 days after treatment as the ratio of plasticity indexto the percentage by weight of particles finer than 2 microns.Generally, a decrease in clay activity is a measure of the clays eitherbeing dissolved at a pH of at least 12.0, or being dissolved andchemically modified by pozzolanic reactions with the soluble calciumions released by calcium hydroxide to form calcium silicate hydrateand/or calcium aluminate hydrates.

As shown in FIG. 11, the 4000 and 10000 ppm calcium hydroxide treatedsamples exhibited the largest decrease in clay activity, with the 4000ppm calcium hydroxide treated sample decreasing from about 5.6 at day 0to about 3.3 at day 7, and the 10000 ppm calcium hydroxide treatedsample decreasing from about 5.0 at day 0 to about 3.0 at day 7. Thecontrol group and 1500 ppm A3338 polymer treated samples exhibitedslight decreases in clay activity, and the 700 ppm alum and 1500 ppmA3338 polymer exhibited an increase in clay activity. The relativelylarge decrease in clay activity exhibited by the 4000 and 10000 ppmtreated samples indicates the effect of pozzolanic reactions, inaccordance with embodiments of the present technology.

FIG. 12 is a graph showing the effect of lime concentration on thespecific gravity of treated tailings over time, in accordance withembodiments of the present technology. For the tests conducted for FIG.12, tailings samples were treated using (a) 0 ppm coagulant orflocculant (i.e., a control group), (b) 4000 ppm calcium hydroxide on awet weight basis, and (c) 10000 ppm calcium hydroxide on a wet weightbasis. The specific gravity was measured for each of the treatedtailings samples at 0 and 28 days after treatment.

As shown in FIG. 12, the control group sample exhibited a slightdecrease in specific gravity from about 2.66 to 2.63 (i.e., a 1%decrease), the 4000 ppm calcium hydroxide treated sample exhibited alarger decrease in specific gravity from about 2.58 to 2.44 (i.e., a 5%decrease), and the 10000 ppm calcium hydroxide treated sample exhibitedan even larger decrease in specific gravity from about 2.61 to 2.37(i.e., a 9% decrease). The calcium silicate hydrates and calciumaluminum hydrates produced via pozzolanic reactions, as discussedelsewhere herein, have higher specific volume and lower specificgravities than clay materials present in untreated tailings.Accordingly, the decrease in specific gravity of the 4000 ppm and 10000ppm calcium hydroxide samples, each of which corresponds to a pH above12.0, may be due to pozzolanic activity and the conversion of clay tocalcium silicate hydrates and/or calcium aluminum hydrates.

FIG. 13 is a graph showing the effect of calcium hydroxide concentrationon particle size of treated tailings, in accordance with embodiments ofthe present technology. For the tests conducted for FIG. 13, tailingssamples were treated using (a) 0 ppm coagulant or flocculant (i.e., acontrol group), (b) 3000 ppm calcium hydroxide on a wet weight basis,(c) 5000 ppm calcium hydroxide on a wet weight basis, and (d) 7000 ppmcalcium hydroxide on a wet weight basis. The diameter of the particlesat which 10% (“d10”), 50% (“d50”), and 90% (“d90”) of the samples arebelow was measured 21 days after treatment.

As shown in FIG. 13, the particle diameters generally increase as theconcentration of calcium hydroxide increases. For example, the d90increases from about 17 microns for the 0 ppm sample, to about 18microns for the 3000 ppm sample, to about 42 for the 5000 ppm sample, toabout 49 for the 7000 ppm sample. Accordingly, the particles size of thetreated tailings samples is directly correlated to the concentration ofcalcium hydroxide added thereto. As also shown in FIG. 13, there is arelatively large increase in the d90 particle diameter from the 3000 ppmto the 5000 ppm sample. Generally speaking, the particle size diameterand/or particle size distribution for a tailings sample may depend inlarge part on the texture of the clays. Accordingly, the relativelylarge increase in the d90 particle diameter may be due to the additionalcalcium hydroxide or calcium ions present in the 5000 ppm and 7000 ppmsamples, which enabled pozzolanic reactions to occur and therebychemically converted the clays (e.g., kaolinite, illite, etc.) tocalcium silicate hydrates and/or calcium aluminum hydrates.

FIG. 14 is a graph showing the effect of temperature over time onundrained peak shear strength of treated tailings over time, inaccordance with embodiments of the present technology. For the testsconducted for FIG. 14, the tailings were heated to (a) an ambientenvironment, (b) 50° C., and (c) 70° C. prior to or during mixing withcalcium hydroxide. The ambient and heated tailings were then provided toa pressure filter. The pressure filtered tailings had about 70% solidscontent by weight. The undrained peak shear strength for each of thepressure filtered tailings was measured at days 0, 7, and 28.

As shown in FIG. 14, at days 0, 7, and 28 the undrained peak shearstrength for the 70° C. sample was higher than that of the 50° C.sample, which was higher than that of the ambient sample. For example,at day 28 the undrained peak shear strength of the (a) ambient samplewas about 7.5 kPa, (b) 50° C. sample was 9.5 kPa, and (c) 70° C. samplewas about 9.8 kPa. As such, the undrained shear strength at day 28,relative to the ambient sample, increased by about 26% for the 50° C.sample and 30% for the 70° C. sample.

IV. Conclusion

It will be apparent to those having skill in the art that changes may bemade to the details of the above-described embodiments without departingfrom the underlying principles of the present disclosure. In some cases,well known structures and functions have not been shown or described indetail to avoid unnecessarily obscuring the description of theembodiments of the present technology. Although steps of methods may bepresented herein in a particular order, alternative embodiments mayperform the steps in a different order. Similarly, certain aspects ofthe present technology disclosed in the context of particularembodiments can be combined or eliminated in other embodiments.Furthermore, while advantages associated with certain embodiments of thepresent technology may have been disclosed in the context of thoseembodiments, other embodiments can also exhibit such advantages, and notall embodiments need necessarily exhibit such advantages or otheradvantages disclosed herein to fall within the scope of the technology.Accordingly, the disclosure and associated technology can encompassother embodiments not expressly shown or described herein, and theinvention is not limited except as by the appended claims.

Throughout this disclosure, the singular terms “a,” “an,” and “the”include plural referents unless the context clearly indicates otherwise.Similarly, unless the word “or” is expressly limited to mean only asingle item exclusive from the other items in reference to a list of twoor more items, then the use of “or” in such a list is to be interpretedas including (a) any single item in the list, (b) all of the items inthe list, or (c) any combination of the items in the list. Additionally,the term “comprising,” “including,” and “having” should be interpretedto mean including at least the recited feature(s) such that any greaternumber of the same feature and/or additional types of other features arenot precluded.

Reference herein to “one embodiment,” “an embodiment,” “someembodiments” or similar formulations means that a particular feature,structure, operation, or characteristic described in connection with theembodiment can be included in at least one embodiment of the presenttechnology. Thus, the appearances of such phrases or formulations hereinare not necessarily all referring to the same embodiment. Furthermore,various particular features, structures, operations, or characteristicsmay be combined in any suitable manner in one or more embodiments.

Unless otherwise indicated, all numbers expressing concentrations, shearstrength, and other numerical values used in the specification andclaims, are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by the present technology. Atthe very least, and not as an attempt to limit the application of thedoctrine of equivalents to the scope of the claims, each numericalparameter should at least be construed in light of the number ofreported significant digits and by applying ordinary roundingtechniques. Additionally, all ranges disclosed herein are to beunderstood to encompass any and all subranges subsumed therein. Forexample, a range of “1 to 10” includes any and all subranges between(and including) the minimum value of 1 and the maximum value of 10,i.e., any and all subranges having a minimum value of equal to orgreater than 1 and a maximum value of equal to or less than 10, e.g.,5.5 to 10.

The disclosure set forth above is not to be interpreted as reflecting anintention that any claim requires more features than those expresslyrecited in that claim. Rather, as the following claims reflect,inventive aspects lie in a combination of fewer than all features of anysingle foregoing disclosed embodiment. Thus, the claims following thisDetailed Description are hereby expressly incorporated into thisDetailed Description, with each claim standing on its own as a separateembodiment. This disclosure includes all permutations of the independentclaims with their dependent claims.

The present technology is illustrated, for example, according to variousaspects described below. Various examples of aspects of the presenttechnology are described as numbered examples (1, 2, 3, etc.) forconvenience. These are provided as examples and do not limit the presenttechnology. It is noted that any of the dependent examples may becombined in any combination, and placed into a respective independentexample. The other examples can be presented in a similar manner.

1. A method for treating a tailings stream from oil sands or miningoperations, comprising:

-   -   providing a tailings stream comprising (i) a solids content of        from 3% to 40% by weight, (ii) bicarbonates, and (iii) a pH less        than 9.0;    -   adding a coagulant comprising calcium hydroxide to the tailings        stream to form a mixture having a pH of at least 11.5 and a        soluble calcium level of no more than 800 mg/L, wherein the pH        and soluble calcium level promote pozzolanic reactions to occur        within the mixture; and    -   after adding the coagulant to the tailings stream, dewatering        the mixture to produce a product having a solids content of at        least 40% by weight, wherein a shear strength of the product        increases over a period of time of at least two days.

2. The method of any one of the previous examples, wherein the shearstrength of the product is an undrained shear strength that, after theperiod of time, is at least 3.0 kilopascals (kPa).

3. The method of any one of the previous examples, wherein the shearstrength of the product is an undrained shear strength that, after theperiod of time, is at least 5.0 kilopascals (kPa).

4. The method of any one of the previous examples, wherein a plasticityindex of the product is less than 30 after the period of time.

5. The method of any one of the previous examples, wherein a residual orremolded shear strength of the product, after the period of time, is atleast 1.5 kilopascals (kPa).

6. The method of any one of the previous examples, wherein the period oftime is at least 7 days, 14 days, 30 days, 60 days, 120 days, or 180days.

7. The method of any one of the previous examples, further comprising,prior to dewatering the mixture, adding a flocculant comprising apolymer to the mixture, wherein the polymer includes polyacrylamide.

8. The method of any one of the previous examples, wherein the coagulantis a first coagulant, the mixture is a first mixture, and the pH of thefirst mixture is no more than 12.0, the method further comprising:

-   -   adding a flocculant comprising a polymer to the first mixture to        form a second mixture, wherein the polymer is configured to bond        with clay of the first mixture while releasing process water;    -   removing or separating the process water from the second        mixture; and    -   adding a second coagulant comprising calcium hydroxide to the        second mixture to form a third mixture having a pH of at least        12.0,

9. The method of example 5, wherein dewatering comprises dewatering thethird mixture to produce release water, the method further comprisingexposing the release water to air such that carbon dioxide of the airreacts with calcium of the release water to form at least one of calciumcarbonate or a buffer comprising bicarbonates.

10. The method of any one of the previous examples, wherein (i) thecoagulant is a first coagulant, (ii) the mixture is a first mixture,(iii) the pH of the first mixture is no more than 12.0, and (iv) thesoluble calcium level of the first mixture is no more than 100 mg/L, themethod further comprising:

-   -   adding a second coagulant comprising calcium hydroxide to the        first mixture to form a second mixture having a pH of at least        12.0 and a soluble calcium level of no more than 800 mg/L,        wherein the pH and soluble calcium level of the second mixture        promote pozzolanic reactions to occur within the second mixture,        and    -   wherein dewatering comprises dewatering the second mixture.

11. The method of example 7, wherein the second mixture comprises clayprovided via the tailings stream, and wherein the pH and the solublecalcium level of the second mixture promotes pozzolanic reactions suchthat the clay is converted to calcium silicate hydrates and/or calciumaluminum hydrates.

12. The method of any one of the previous examples, wherein thepozzolanic reactions occurring within the mixture do not produce gaseouscarbon dioxide as a byproduct.

13. The method of any one of the previous examples, wherein thecoagulant is a slurry comprising from 1% to 20% calcium hydroxide.

14. The method of any one of the previous examples, further comprisingallowing the mixture to settle over a predetermined period of time of atleast two days.

15. The method of any one of the previous examples, wherein the calciumhydroxide comprises particles having a specific surface area of at least25 m²/g.

16. The method of any one of the previous examples, wherein particles ofthe product, before the period of time, comprise a first averageparticle size, and wherein the particles of the product, after theperiod of time, comprise a second average particle size larger than thefirst average particle size.

17. The method of any one of the previous examples, wherein the product,before the period of time, comprises particles having a d90 less than 20microns, and wherein the d90 of the particle, after the period of time,is greater than 20 microns.

18. The method of any one of the previous examples, wherein adding thecoagulant comprises adding at least 3,000 ppm calcium hydroxide on a wetweight basis.

19. The method of any one of the previous examples, wherein adding thecoagulant comprises adding at least 8,000 ppm calcium hydroxide on a wetweight basis.

20. The method of any one of the previous examples, wherein the shearstrength is undrained shear strength or undrained peak shear strength.

21. The method of any one of the previous examples, wherein the shearstrength is undrained shear stress or undrained peak shear stress.

22. The method of any one of the previous examples, wherein the shearstrength is undrained shear strength, the method further comprising,after the dewatering, shearing the mixture by pumping the product to acontainment area, wherein the shear strength of the product aftershearing and after the period of time is at least 3.0 kilopascals.

23. The method of any one of the previous examples, wherein the tailingsstream comprises fly ash tailings, oil sands tailings or miningtailings.

24. The method of any one of the previous examples, wherein the productcomprises a solids content of at least 65% by weight.

25. The method of any one of the previous examples, further comprisingheating the product to a temperature of at least 50° C. for the periodof time.

26. The method of any one of the previous examples, wherein the productis a cake.

27. A system for treating tailings from oil sands or mining operations,comprising:

-   -   a mixer or in-line mixing area configured to—        -   receive (i) tailings comprising clay and bicarbonates            and (ii) a coagulant comprising calcium hydroxide, and        -   mix the tailings and coagulant to form a first mixture            comprising a pH of at least 11.5 and a soluble calcium level            of no more than 800 mg/L; and    -   a dewatering device downstream of the thickener vessel and        configured to dewater the first mixture to produce a product        having a shear strength that increases over a period of time of        at least two days.

28. The system of any one of the previous examples, wherein the shearstrength of the product, after the period of time, is at least 3.0kilopascals.

29. The system of any one of the previous examples, wherein the shearstrength of the product is an undrained shear strength that, after theperiod of time, is at least 5.0 kilopascals.

30. The system of any one of the previous examples, wherein a plasticityindex of the product is less than 30 after the period of time.

31. The system of any one of the previous examples, wherein a residualor remolded shear strength of the product, after the period of time, isat least 1.5 kilopascals.

32. The system of any one of the previous examples, wherein the periodof time is at least 7 days, 14 days, 30 days, 60 days, 120 days, or 180days.

33. The system of any one of the previous examples, further comprising athickener vessel downstream of the mixer and configured to (i) receivethe first mixture and (ii) produce process water and a second mixture,wherein the dewatering device is configured to dewater the secondmixture, and wherein the thickener vessel is configured to receive aflocculant comprising a polyacrylamide polymer.

34. The system of any one of the previous examples, wherein (i) thecoagulant is a first coagulant, (ii) the pH of the first mixture is nomore than 12.0, (iii) the mixer is a first mixer, and (iv) the solublecalcium level of the first mixture is no more than 100 mg/L, the systemfurther comprising:

-   -   a thickener vessel downstream of the first mixer and configured        to (i) receive the first mixture and (ii) produce process water        and a second mixture; and    -   a second mixer downstream of the thickener vessel and configured        to (i) receive the second mixture and a second coagulant        comprising calcium hydroxide, and (ii) mix the second mixture        and second coagulant to produce a third mixture having a pH of        at least 12.0,    -   wherein the dewatering device is configured to dewater the third        mixture to produce the product.

35. The system of example 34, wherein dewatering the third mixtureproduces release water, the system further comprising a containment areaconfigured to (i) receive the release water, and (ii) expose the releasewater to air such that carbon dioxide of the air reacts with alkalinecalcium of the release water to form at least one of calcium carbonateor a buffer comprising bicarbonates or carbonates on an outer surface ofthe product.

36. The system of any one of examples 34 or 35, wherein the secondmixture comprises clay provided via the tailings, and wherein the pH andthe soluble calcium level of the third mixture promotes pozzolanicreactions such that the clay is converted to silicate hydrates and/oraluminum hydrates.

37. The system of any one of the previous examples, wherein thecoagulant is a slurry comprising 1% to 10% calcium hydroxide.

38. The system of any one of the previous examples, wherein the calciumhydroxide comprises particles having a specific surface area of at least25 m²/g.

39. The system of any one of the previous examples, wherein particles ofthe product, before the period of time, comprise a first averageparticle size, and wherein the particles, after the period of time,comprise a second average particle size larger than the first averageparticle size.

40. The system of any one of the previous examples, wherein the product,before the period of time, comprises particles such that a d90 of theparticles is 20 microns, and wherein the d90 of the particle, after theperiod of time, is greater than 20 microns.

41. The system of any one of the previous examples, wherein thecoagulant comprises at least 3,000 mg/L calcium hydroxide.

42. The system of any one of the previous examples, wherein thecoagulant comprises at least 8,000 mg/L calcium hydroxide.

43. The system of any one of the previous examples, wherein the productcomprises a solids content of at least 65% by weight.

44. The system of any one of the previous examples, further comprisingheating the product to a temperature of at least 50° C. for the periodof time.

45. The system of any one of the previous examples, wherein the productis a cake.

46. A method for treating tailings from oil sands or mining operations,comprising:

-   -   providing tailings comprising (i) a solids content of from 3% to        40% by weight, and (ii) bicarbonates;    -   adding a first coagulant comprising calcium hydroxide to the        tailings to form a first mixture, the first mixture having (i) a        pH within a range of 11.5 to 12.0 and (ii) a soluble calcium        level of no more than 200 mg/L;    -   adding a flocculant comprising a polymer to the first mixture to        form a second mixture;    -   adding a second coagulant comprising calcium hydroxide to the        second mixture to form a third mixture, the third mixture having        a pH of at least 12.0 and a soluble calcium level of no more        than 800 mg/L, wherein the pH and soluble calcium level promote        pozzolanic reactions to occur within the third mixture; and    -   after adding the second coagulant to the tailings, dewatering        the third mixture to produce a product having a solids content        of at least 40% by weight and a release water having a solids        content less than 10% by weight, wherein an undrained shear        strength of the product continually increases over a period of        time of at least two days.

47. The method of any one of the previous examples, wherein the product,after the period of time, includes a plasticity index of less than 30.

48. The method of any one of the previous examples, wherein the thirdmixture comprises clay, and wherein the pH and the soluble calcium levelof the third mixture promotes pozzolanic reactions such that the clay isconverted to calcium silicate hydrates and/or calcium aluminum hydrates.

49. The method of any one of the previous examples, wherein theflocculant is added to the first mixture in a vessel, the method furthercomprising removing process water from the vessel prior to adding thesecond coagulant to the second mixture.

50. The method of any one of the previous examples, wherein the pH ofthe first mixture is no more than 12.0.

We claim:
 1. A method for treating a tailings stream, comprising:providing a tailings stream comprising (i) a solids content of from 3%to 40% by weight, (ii) bicarbonates, and (iii) a pH less than 9.0;adding a coagulant comprising calcium hydroxide to the tailings streamto form a mixture having a pH of at least 12.0 and a soluble calciumlevel of no more than 800 mg/L, wherein the pH and soluble calcium levelpromote pozzolanic reactions to occur within the mixture; and afteradding the coagulant to the tailings stream, dewatering the mixture toproduce a product having a solids content of at least 40% by weight,wherein shear strength of the product increases over a period of time ofat least two days to at least 1.5 kilopascals.
 2. The method of claim 1,wherein the shear strength of the product is an undrained shear strengththat, after the period of time, is at least 5.0 kilopascals (kPa). 3.The method of claim 1, wherein the period of time is at least 14 days.4. The method of claim 1, further comprising, prior to dewatering themixture, adding a flocculant comprising a polymer to the mixture,wherein the polymer includes polyacrylamide.
 5. The method of claim 1,wherein dewatering comprises dewatering the mixture to produce releasewater, the method further comprising exposing the release water to airsuch that carbon dioxide of the air reacts with calcium of the releasewater to form at least one of calcium carbonate or a buffer comprisingbicarbonates.
 6. The method of claim 1, wherein the mixture comprisesclay provided via the tailings stream, and wherein the pH and thesoluble calcium level of the mixture promotes pozzolanic reactions suchthat the clay is converted to calcium silicate hydrates and/or calciumaluminum hydrates.
 7. The method of claim 1, wherein the pozzolanicreactions occurring within the mixture do not produce gaseous carbondioxide as a byproduct.
 8. The method of claim 1, wherein the calciumhydroxide comprises particles having a specific surface area of at least25 m²/g.
 9. The method of claim 1, wherein the product, before theperiod of time, comprises particles having a d90 less than 20 microns,and wherein the d90 of the particle, after the period of time, isgreater than 20 microns.
 10. The method of claim 1, wherein adding thecoagulant comprises adding at least 8,000 ppm calcium hydroxide.
 11. Themethod of claim 1, wherein the shear strength is an undrained shearstrength or undrained peak shear strength.
 12. A method for treatingtailings, comprising: obtaining tailings comprising clay andbicarbonates; combining at least a portion of the tailings with a firstcoagulant comprising calcium hydroxide to form a first mixture having apH of at least 11.5 to no more than 12.0; combining a second coagulantcomprising calcium hydroxide with the first mixture to form a secondmixture having a pH of at least 12.0 and a soluble calcium level of nomore than 800 mg/L, wherein the pH and soluble calcium level promotepozzolanic reactions to occur within the second mixture; and dewateringthe second mixture to produce a product having a shear strength thatincreases over a period of time of at least two days to be at least 1.5kilopascals.
 13. The method of claim 12, wherein combining at least aportion of the tailings with the first coagulant occurs in a first mixerupstream of a thickener vessel, and combining the second coagulant withthe first mixture occurs within a second mixer downstream of thethickener vessel.
 14. The method of claim 13, further comprisingseparating, via the thickener vessel, the first mixture into releasewater and the second mixture.
 15. The method of claim 12, furthercomprising, prior to combining the second coagulant with the firstmixture, removing or separating water from the first mixture.
 16. Themethod of claim 12, wherein the first mixture has a soluble calciumlevel of no more than 100 mg/L.
 17. The method of claim 12, furthercomprising adding a flocculant comprising a polymer to the firstmixture, the polymer being configured to bond with clay of the firstmixture.
 18. A system for treating tailings, comprising: a mixer orinline mixing area configured to receive (i) tailings comprising clayand bicarbonates and (ii) a coagulant comprising calcium hydroxide, andmix the tailings and coagulant to form a first mixture comprising a pHof at least 12.0 and a soluble calcium level of no more than 800 mg/L;and a dewatering device configured to dewater the mixture to produce aproduct having a shear strength that increases over a period of time ofat least two days to at least 1.5 kilopascals.
 19. The system of claim18, wherein the shear strength of the product, after the period of time,is at least 5.0 kilopascals.
 20. The system of claim 18, whereindewatering the mixture produces release water, the system furthercomprising a containment area configured to (i) receive the releasewater, and (ii) expose the release water to air such that carbon dioxideof the air reacts with alkaline calcium of the release water to form atleast one of calcium carbonate or a buffer comprising bicarbonates orcarbonates.
 21. The system of claim 18, wherein the coagulant is aslurry comprising 1% to 10% calcium hydroxide.
 22. The system of claim18, wherein particles of the product, before the period of time,comprise a first average particle size, and wherein the particles, afterthe period of time, comprise a second average particle size larger thanthe first average particle size.